Research will be continued to gain further understanding of the biochemistry of 24(S),25-epoxycholesterol and other oxygenated sterols. Recent evidence has been obtained that this epoxy sterol occurs naturally in human liver tissue and in cultured cells, that it represses HMG-CoA reductase activity, and that it binds to the cytosolic oxysterol binding protein. These findings suggest that 24(S),25-epoxycholesterol is a natural regulator of cholesterol formation. Further efforts will be directed toward gaining direct evidence for such a regulatory role for this epoxide and other oxysterols. Included will be determination of the incidence of epoxy sterols and other oxysterols in additional media selected to elucidate a regulatory role. Evidence will be sought that the concentration of cellular oxysterols changes as a function of a change in HMG-CoA reductase activity. The biosynthesis of 25-hydroxycholesterol will be elucidated and the metabolic fate of that compound and of 24(S),25-epoxycholesterol will be determined. Other experiments will be conducted to determine if 24(S),25-epoxycholesterol or other substances derived from squalene dioxide have additional diochemical importance. Effort will also be expended to develop photoaffinity labels to assist in characterization of two important enzymes involved in sterol biochemistry, the oxysterol binding protein and oxidosqualene cyclase. Additional prospective late-stage inhibitors of cholesterol biosynthesis will be prepared and evaluated. Attempts will be made to develop a radioimmunoassay for mevalonic acid. All of these planned experiments have as their fundamental goal a better understanding of the mechanism of cholesterol regulation. Such an understanding will eventually be important in achieving control of atherosclerosis, and the results may have implications for control of tumor cell growth as well.